Communicate between two windows forms in C#

Question

I have two forms, one is the main form and the other is an options form. So say for example that the user clicks on my menu on the main form: Tools -> Options

Answer 1

Properties is one option, shared static class - another option, events - another option...

Answer 2

You can have a function in Form B like so:

public SettingsResults GetNewSettings()
{
    if(this.ShowDialog() == DialogResult.Ok)
    {
         return new SettingsResult { ... };
    }
    else
    {
         return null;
    }
}

And you can call it like this:

...
using(var fb = new FormB())
{
    var s = fb.GetNewSettings();
    ...
    // Notify other parts of the application that settings have changed.
}

Answer 3

In the comments to the accepted answer, Neeraj Gulia writes:

This leads to tight coupling of the forms Form1 and Form2, I guess instead one should use custom events for such kind of scenarios.

The comment is exactly right. The accepted answer is not bad; for simple programs, and especially for people just learning programming and trying to get basic scenarios to work, it's a very useful example of how a pair of forms can interact.

However, it's true that the coupling that example causes can and should be avoided, and that in the particular example, an event would accomplish the same thing in a general-purpose, decoupled way.

Here's an example, using the accepted answer's code as the baseline:

Form1.cs:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        Form2 frm = new Form2();

        frm.Button1Click += (sender, e) => Lbl.Text = ((Form2)sender).Message;

        frm.Show();
    }
}

The above code creates a new instance of Form2, and then before showing it, adds an event handler to that form's Button1Click event.

Note that the expression (sender, e) => Lbl.Text = ((Form2)sender).Message is converted automatically by the compiler to a method that looks something similar to (but definitely not exactly like) this:

private void frm_Message(object sender, EventArgs e)
{
    Lbl.Text = ((Form2)sender).Message;
}

There are actually lots of ways/syntaxes to implement and subscribe the event handler. For example, using an anonymous method as the above, you don't really need to cast the sender parameter; instead you can just use the frm local variable directly: (sender, e) => Lbl.Text = frm.Message.

Going the other way, you don't need to use an anonymous method. You could in fact just declare a regular method just like the compiler-generated one I show above, and then subscribe that method to the event: frm.Button1Click += frm_Message; (where you have of course used the name frm_Message for the method, just as in my example above).

Regardless of how you do it, of course you will need for Form2 to actually implement that Button1Click event. That's very simple…

Form2.cs:

public partial class Form2 : Form
{
    public event EventHandler Button1Click;

    public string Message { get { return txtMessage.Text; } }

    public Form2()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        EventHandler handler = Button1Click;

        if (handler != null)
        {
            handler(this, EventArgs.Empty);
        }
    }
}

In addition to the event, I've also declared a property Message that exposes the Text property (and only the Text property, and only as read-only in fact) of the txtMessage control. This allows the subscriber to the event to get the value and do whatever it needs to with it.

Note that all that the event does is to alert the subscriber that the button has in fact been clicked. It's up to the subscriber to decide how to interpret or react to that event (e.g. by retrieving the value of the Message property and assigning it to something).

Alternatively, you could in fact deliver the text along with the event itself, by declaring a new EventArgs sub-class and using that for the event instead:

public class MessageEventArgs : EventArgs
{
    public string Message { get; private set; }

    public MessageEventArgs(string message)
    {
        Message = message;
    }
}

public partial class Form2 : Form
{
    public event EventHandler<MessageEventArgs> Button1Click;

    public Form2()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        EventHandler handler = Button1Click;

        if (handler != null)
        {
            handler(this, new MessageEventArgs(txtMessage.Text));
        }
    }
}

Then the subscriber can just retrieve the message value directly from the event object:

frm.Button1Click += (sender, e) => Lbl.Text = e.Message;

The important thing note in all of the above variations is that at no point does the class Form2 need to know anything about Form1. Having Form1 know about Form2 is unavoidable; after all, that's the object that will create a new Form2 instance and use it. But the relationship can be asymmetrical, with Form2 being usable by any object that needs the features it offers. By exposing the functionality as an event (and optionally with a property), it makes itself useful without limiting its usefulness to only the Form1 class.

Answer 4

This is probably sidestepping your problem a little bit, but my settings dialog uses the Application Settings construct. http://msdn.microsoft.com/en-us/library/k4s6c3a0.aspx

I can't find a good example that's similar to how I do it (which is actually having an actual class+object), but this covers another way of doing it:

Reading default application settings in C#

Answer 5

There are lots of ways to perform communication between two Forms. Some of them have already been explained to you. I am showing you the other way around.

Assuming you have to update some settings from the child form to the parent form. You can make use of these two ways as well :

1. Using System.Action (Here you simply pass the main forms function as the parameter to the child form like a callback function)
2. OpenForms Method ( You directly call one of your open forms)

Using System.Action

You can think of it as a callback function passed to the child form.

// -------- IN THE MAIN FORM --------

// CALLING THE CHILD FORM IN YOUR CODE LOOKS LIKE THIS
Options frmOptions = new Options(UpdateSettings);
frmOptions.Show();

// YOUR FUNCTION IN THE MAIN FORM TO BE EXECUTED
public void UpdateSettings(string data)
{
   // DO YOUR STUFF HERE
}

// -------- IN THE CHILD FORM --------

Action<string> UpdateSettings = null;

// IN THE CHILD FORMS CONSTRUCTOR
public Options(Action<string> UpdateSettings)
{
    InitializeComponent();
    this.UpdateSettings = UpdateSettings;
}

private void btnUpdate_Click(object sender, EventArgs e)
{
    // CALLING THE CALLBACK FUNCTION
    if (UpdateSettings != null)
        UpdateSettings("some data");
}

OpenForms Method

This method is easy (2 lines). But only works with forms that are open. All you need to do is add these two lines where ever you want to pass some data.

Main frmMain = (Main)Application.OpenForms["Main"];
frmMain.UpdateSettings("Some data");

Answer 6

The best way to deal with communication between containers is to implement an observer class

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. (Wikipedia)

the way i do this is creating an Observer class, and inside it write something like this:

1    public delegate void dlFuncToBeImplemented(string signal);
2    public static event dlFuncToBeImplemented OnFuncToBeImplemented;
3    public static void FuncToBeImplemented(string signal)
4    {
5         OnFuncToBeImplemented(signal);
6    }

so basically: the first line says that there would be a function that somebody else will implement

the second line is creating an event that occurs when the delegated function will call

and the third line is the creation of the function that calls the event

so in your UserControl, you should add a function like this:

private void ObserverRegister()//will contain all observer function registration
{
    Observer.OnFuncToBeImplemented += Observer_OnFuncToBeImplemented;
    /*and more observer function registration............*/
}


void Observer_OnFuncToBeImplemented(string signal)//the function that will occur when  FuncToBeImplemented(signal) will call 
{
    MessageBox.Show("Signal "+signal+" received!", "Atention!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
}

and in your Form you should do something like:

public static int signal = 0;

public void button1_Click(object sender, EventArgs e)
{
      Observer.FuncToBeImplemented(signal);//will call the event in the user control
}

and now, you can register this function to a whole bunch of other controls and containers and they will all get the signal

I hope this would help :)